In recent years, an organic semiconductor using an organic substance has attracted attention as a semiconductor for a TFT.
Generally, organic substances used in an organic semiconductor have such advantages that not only they can be formed into a thin film by a relatively easy film-forming method such as spin coating and vacuum vapor deposition, but also they can produce a TFT at low temperatures as compared with a conventional TFT which is prepared using amorphous or polycrystalline silicon. Processing at low temperatures enables formation of a film on a plastic substrate having a low heat resistance. As a result, various effects such as a decreased display weight or a reduced cost, as well as diversified applications due to flexibility of the plastic substrate can be expected.
As the organic substance constituting an organic semiconductor, an organic substance with a large carrier mobility, which is used in a TFT or the like, is used. Examples include a single crystal of acene such as anthracene, tetracene, and pentacene. An organic semiconductor formed of such a substance is reported to have a carrier mobility of about 1 (cm2V−1s−1).
Using such an organic semiconductor in a TFT encounters a problem that control of the conductivity type of a channel by doping with impurities is difficult as compared with the case where amorphous or polycrystalline silicon is used in a TFT. Therefore, in order to control the conductivity type of a channel in a transistor where a plurality of TFTs are formed on a substrate in a complementary transistor circuit, for example, an organic substance used in the organic semiconductor is selected appropriately, and an n-channel type TFT and a p-channel type TFT are fabricated separately.
However, since the n-channel type TFT and the p-channel type TFT are fabricated separately using different organic substances, such a circuit is difficult to be put into a practical use due to a complicated production process and an increased production cost.
Under such circumstances, study has been made on a semiconductor provided with the so-called ambipolar characteristics capable of transporting both holes and electrons. Unlike the case where only a monopolar semiconductor is used, such an ambipolar semiconductor enables a circuit composed of a plurality of TFTs to act as both an n-channel type TFT and a p-channel type TFT using only a single organic substance. Accordingly, separate patterning of an n-channel type TFT and a p-channel type TFT becomes unnecessary. As a result, production process is simplified and a significantly reduced production cost can be expected.
As for the technology of an ambipolar semiconductor using an organic substance, a single crystal of pentacene with a high degree of purity is known to function as an ambipolar semiconductor (Non-Patent Document 1). An organic semiconductor formed of a single crystal of the above-mentioned other organic substances than pentacene above or an organic semiconductor formed of a single crystal of pentacene with a low degree of purity is monopolar, and functions either p-type (hole transporting) or n-type (electron transporting). In contrast, an organic semiconductor formed of a single crystal of pentacene with a high degree of purity functions both as a p-type semiconductor and an n-type semiconductor.
As examples of the conventional ambipolar semiconductor technology, the following can be given. A voltage control layer formed of a silane compound or the like is stacked on an organic semiconductor layer to allow the semiconductor to exhibit ambipolar characteristics (Patent Document 1). A carbon nanotube is combined with an organic semiconductor to allow the organic semiconductor to exhibit ambipolar characteristics (Patent Document 2). Particles in the form of a thin film obtained by oxidation of carbon black are incorporated in an organic semiconductor to allow the organic semiconductor to exhibit ambipolar characteristics (Patent Document 3). An organic substance with n-type characteristics and an organic substance with p-type characteristics are incorporated in combination in an organic semiconductor to allow the organic semiconductor to exhibit ambipolar characteristics (Patent Document 4). A mixture of an organic substance with n-type characteristics and an organic substance with p-type characteristics is used in an organic semiconductor to allow the organic semiconductor to exhibit ambipolar characteristics (Patent Document 5). A semiconductor device prepared using an amorphous oxide and is provided with an n-type region and a p-type region (Patent Document 6).
Technologies of applying a semiconductor device such as a TFT in a CMOS circuit are described in Patent Documents 7 and 8. Patent Documents 4 and 5 describe application of a semiconductor device such as a TFT prepared by using an ambipolar semiconductor is used to a display, a screen, an inverter, or the like.
Patent Document 1: JP-A-2005-26872
Patent Document 2: JP-A-2005-150410
Patent Document 3: JP-A-2004-95790
Patent Document 4: JP-A-2005-209455
Patent Document 5: JP-B-2006-518938
Patent Document 6: JP-A-2006-165532
Patent Document 7: JP-A-2001-177109
Patent Document 8: JP-A-2002-26336
Non-Patent Document: J. H. Schon, et al, Science Vol. 287, 1022 (2000)
In the above-mentioned Non-Patent Document 1, almost no substances other than pentacene are reported as an organic substance which exhibits ambipolar characteristics alone without combining with other substances. Therefore, only a limited material selection is permitted, which may make circuit design or the like difficult. In addition, pentacene exhibits ambipolar characteristics only when it has a high degree of purity. Normal pentacene exhibits only p-type characteristics, which may make production of a semiconductor device difficult and costly.
Therefore, like the technologies described in Patent Documents 4 and 5, an ambipolar semiconductor is fabricated using both an organic substance having n-type characteristics and an organic substance having p-type characteristics in order not to depend on the characteristics of a single substance, and such an ambipolar semiconductor is used in a semiconductor device such as a complementary transistor or a light-emitting element. However, the technologies disclosed in Patent Documents 4 and 5 have such disadvantages that operation of the semiconductor and the semiconductor device using thereof tends to be unstable due to the combined use of an organic substance having n-type characteristics and an organic substance having p-type characteristics.
The invention has been made in view of the above problems, and an object thereof is to provide a semiconductor which can be fabricated at a low cost due to the use of an organic semiconductor, can exhibit ambipolar characteristics, and is capable of being used widely due to stable operation, a semiconductor device using the semiconductor, and a complementary transistor circuit using the semiconductor device.